The existing art describes FM antenna amplifiers that are used for mobile reception in order to improve radio reception if antennas either are poorly adapted and/or possess low antenna efficiency. In the context of reception close to the transmitter, very high levels occur in the antenna amplifier and also in the car radio downstream. These high levels cause an intermodulation which interferes with reception if the frequency of an intermodulation product coincides with the reception channel that is selected. To prevent these kinds of interference, antenna amplifiers having a level setting element at the amplifier input are used. The result, when the preset control threshold is reached, is that the level at the output of the control element and the intermodulation products no longer rise even if the intermodulation products continue to increase, i.e. continue to rise. This applies, however, only to the control range implemented within the circuit. Above the control range, the output level rises to the same degree as the input level.
Amplifiers having these aforementioned features are known, and are exported e.g. to the United States. Such amplifiers are used in particular in motor vehicles that are intended for export to the United States. The known amplifiers have the following substantial disadvantages:
The magnitude of the controlled output level depends greatly on the ambient temperature. In addition, the control range—i.e. the range within which the output level remains constant as the input level rises—is insufficiently wide.
The result of the dependency on ambient temperature is that in a cold winter, when temperatures of e.g. −40° C. can occur at the antenna amplifier, the controlled output level will have a value different from that in a hot summer, when the temperature at the antenna amplifier can rise to approx. 90° C. The behavior of the reception system when passing through strong signal areas therefore depends on the season.
The inadequate control range means that the input level is damped by only a few decibels in the adjusting element. This yields almost no improvement in reception, since intermodulation is only slightly suppressed.
FIG. 5 shows the circuit diagram of an antenna amplifier according to the existing art in which the output level is regulated. The antenna amplifier has a signal amplifier V, a control circuit RS that controls a PIN diode, and an antenna adapter A having an input HF-E for the antenna signal and an output HF-A for the amplified signal. A small portion of the power of the output signal of signal amplifier V is diverted to a rectifier Dg, Rg, Cg. In control circuit RS, a rectified voltage Ub is amplified in a first operational amplifier stage OP1 and integrated in second stage OP2. The output signal controls a PIN diode PIN whose anode is connected to the positive voltage and has a high-frequency connection to ground by way of a capacitor. Diode PIN short-circuits the HF voltage to ground when it is completely conductive. The temperature dependency occurs in diode Dg of the HF rectifier. To ensure that the wide fluctuation range in the conducting-stage voltage that occurs here does not result in malfunctions, the value of the control threshold is selected so that the resulting rectified voltage is high compared to the temperature-related fluctuation range of the conducting-state voltage. With this known amplifier device, the object is achieved by the fact that the output voltage is picked off at the high point of a series oscillator circuit Ls/Cs, where the voltage is higher (because of the high impedance) than directly at the amplifier output. A high voltage is present at rectifier diode Dg, however, and causes an intermodulation problem there.
The size of the control range and thus the maximum damping achievable by way of PIN diode PIN depends here on the antenna impedance, which together with PIN diode PIN forms a voltage divider. The ratio of this voltage divider is high in order to yield a high level of antenna signal damping. This is achievable if the antenna's source impedance is high, but that impedance varies greatly in frequency-dependent fashion. As the impedance fluctuates, the damping frequency response that results at the maximum PIN diode current is also inhomogeneous.